refactor: move act under internal

The runner is an application, not a library. `act/model` and
`act/exprparser` were the last packages anything outside this repository
consumed and they now live in actionslib, so nothing needs the rest of
`act` to be importable, and keeping it importable invites the coupling
that was just removed.

Import paths only, the files are unchanged.

Assisted-by: Codet:GPT-5.1-Codex

internal/act/runner/testdata/secrets/.env

@@ -0,0 +1,2 @@
+HELLO=WORLD
+MULTILINE_ENV="foo\nbar\nbaz"

internal/act/runner/testdata/secrets/.secrets

@@ -0,0 +1,3 @@
+MY_SECRET=top-secret
+MULTILINE_SECRET="foo\nbar\nbaz"
+JSON_SECRET={"foo": "bar"}

internal/act/runner/testdata/secrets/push.yml

@@ -0,0 +1,17 @@
+name: basic
+on: push
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          echo '${{secrets.MY_SECRET}}' | grep 'top-secret'
+      - run: |
+          echo "${{secrets.MULTILINE_SECRET}}" | wc -l | grep 3
+      - run: |
+          echo '${{secrets.JSON_SECRET}}' | grep "{\"foo\": \"bar\"}"
+      - run: |
+          echo '${{env.HELLO}}' | grep "WORLD"
+      - run: |
+          echo "${{env.MULTILINE_ENV}}" | wc -l | grep 3